1. Field of the Invention
This invention relates to a fuel injection control system for internal combustion engines, which controls an amount of fuel to be injected so as to compensate for an amount of fuel adherent to the wall surface of the intake pipe of the engine.
2. Prior Art
While most of fuel injected by a fuel injection valve into the intake pipe of an internal combustion engine is directly supplied into the combustion chamber of the engine, the remainder of fuel adheres to the wall surface of the intake pipe. A fuel injection amount control system is conventionally known, which estimates an amount of fuel to adhere to the wall surface of the intake pipe and an amount of fuel to be carried off the wall surface into the combustion chamber due to evaporation and other factors (carried-off fuel amount), and then determines an appropriate amount of fuel to be injected (fuel injection amount), by taking into account these estimated amounts of fuel (adherent fuel-dependent correction of the fuel injection amount).
Further, a method of carrying out the adherent fuel-dependent correction is known from Japanese Laid-Open Patent Publication (Kokai) No. 62-218633, in which an adherent fuel amount is calculated during stoppage of the engine, and the adherent fuel-dependent correction is carried out at the next start of the engine, based on the adherent fuel amount calculated during the stoppage of the engine. There is also known a method of setting an initial value of the adherent fuel amount at the start of the engine, according to the temperature of the engine, for example, from Japanese Laid-Open Patent Publication (Kokai) No. 62-223429. These known methods attempt to prevent the inconvenience that at the restart of the engine when only a short time period has elapsed from the termination of the last engine operation, the adherent fuel-dependent correction is effected to an excessive degree in spite of the fact that almost no fuel remains adherent to the wall surface of the intake pipe which still has a high temperature at the restart of the engine.
However, adhesion-dependent correction parameters such as a direct supply ratio which is the ratio of a fuel amount directly drawn into the combustion chamber to the whole fuel amount injected in a cycle, and a "carried-off" time constant which corresponds to a time delay with which fuel adhering to the intake pipe wall surface is carried off into the combustion chamber, are estimated when the engine is operating in a stable condition. More specifically, these parameters are estimated from a response characteristic of the air-fuel ratio of exhaust gases which is obtained by stepwise changing the fuel injection amount in an engine condition where the intake pipe pressure and the engine rotational speed are constant. However, the parameter values are estimated to steady values based on the engine coolant temperature, etc., which are not values quantified based on actually measured values. Therefore, the estimated parameter values obtained at the start of the engine are low in accuracy, which leads to low accuracy of the adherent fuel-dependent correction during the start of the engine. As a result, if the adherent fuel-dependent correction is carried out during the start of the engine, the fuel injection amount may be sometimes corrected to an excessive degree, which unfavorably causes the air-fuel ratio of a mixture supplied to the engine to deviate from a desired value during the start of the engine or immediately after the start of the engine.
To overcome deviation of the air-fuel ratio from the desired value, if the adherent fuel-dependent correction is inhibited during the start of the engine and the correction is started immediately after the start of the engine, e.g. after the engine rotational speed exceeds a predetermined value, the carried-off fuel amount will be calculated to 0 immediately after the start of the engine, and consequently the fuel injection amount immediately after the start of the engine exceeds a required fuel amount, resulting in overriching of the air-fuel ratio of the mixture.